The invention concerns a process for the generation of ultra-short laser light pulses, in particular a process for stabilizing of the operation of a pulse laser, and a process for the production of highly accurate optical frequencies, and a laser device for the generation of ultra-short light pulses, in particular a frequency stabilized pulse laser, and applications of such a laser device in spectroscopy, time or frequency measurement technology and communications technology.
The production of ultra-short laser light pulses (light pulses with a typical pulse duration in ns- to fs- range) known since the 1970s is based on so-called mode synchronization. In a laser medium many fundamental oscillations with different frequencies may be generated, given sufficient bandwidth of the laser transition in the resonator. If a suitable mechanism is provided to produce a stable phase relationship between the fundamental oscillations (mode synchronization), short light pulses with a time separation are produced, being equal to the quotient of the doubled resonator length and circulation velocity of the pulses, and corresponding in their spectroscopic composition to the optical frequencies generated in the resonator and contributing to the formation of the pulses.
A so-called frequency comb is obtained from a Fourier transformation of the course of intensity of the laser beam""s pulse form, formed through xcex4-type functions of optical frequencies contributing to each pulse and whose envelope is within the bandwidth of the laser transition in the laser medium. Essentially, the width of the envelope is inversely proportional to the pulse duration. An example for such a frequency comb is shown schematically in FIG. 5. Each contributing frequency to such a frequency comb is designated as mode M. Corresponding to the longitudinal laser modes, the frequency separations of the elements of the frequency comb are even number multiples of the pulse repetition frequency fr=xcfx84xe2x88x921 (repetition rate) . The comb structure of fspulses in the frequency space is described, for example, in xe2x80x9cFemtosecond Laser Pulsesxe2x80x9d (C. Rulliere, ed., Springer Publications, Berlin 1998).
Since the pulse repetition frequency fr depends on the resonator length, deviations of the ideally stable mode intervals occur even in the presence of most insignificant instabilities of the resonator. Techniques for the stabilization of the resonator length are known, which suppress changes in the mode intervals. To this end, a movable resonator end mirror in the direction along the length of the resonator is installed and adjusted in the presence of a mode shift through the use of a control loop. This conventional stabilization method is insufficient for the present-day accuracy requirements in the use of spectroscopy or time measurement technology.
J. N. Eckstein et al. (see xe2x80x9cPhysical Review Lettersxe2x80x9d, Vol. 40, 1978, p. 847 et seq.) recognized that the row of the modes would be suitable as a scale for frequency calibration At the same time the insufficient stability of the pulse laser and noise-related shifts in mode frequencies were pointed out. It was observed that the shifts continued to occur in spite of the stabilization of the resonator length. According to L. Xu et al. In xe2x80x9cOptics Lettersxe2x80x9d, Vol. 21, 1996, p. 2008 et seq., the cause is the group velocity of a pulse, which determines the circulation time in the resonator and therefore the repetition frequency, as a rule does not correspond to the phase velocity of the individual modes. The modes separated by the even number multiples of the repetition frequency cannot be represented in their absolute frequency position through even number multiples (n) of the repetition frequency fr, but rather through the sum (nxc2x7fr+fp) of nxc2x7repetition frequency fr and a so-called phase slip frequency fp which has the same value for all modes, corresponding to the quotient of the respective phase differences from pulse to pulse through the circulation time (2Π)xcfx84. A determination of these phase differences is until now not available, so that the use of pulse lasers for measuring purposes or as generators of optical frequencies is limited.
In the following, two areas in which there is an interest in highly accurate frequencies will be described. The first application concerns frequency measurement generally, in particular making time or frequency standards available. The second application lies in the area of spectroscopy, in particular the measurement of atomic electronic energy transitions.
A widely used time standard is given through the so-called cesium atom clock with a base frequency of 9.2 GHz. The time measurement is carried out through a direct counting out of the base oscillations, which is presently possible with a degree of accuracy of for example 10xe2x88x924. Significantly higher relative accuracy up to the magnitude of 10xe2x88x9218 are expected from optical frequency standards, for example on the basis of cooled ions in field cages (see, e.g., M. Roberts et al. In xe2x80x9cPhysical Review Lettersxe2x80x9d, Vol. 78, 1997, pp. 1876 et seq.) or from the extremely narrow atomic resonances such as that of the 1S-2S-transition of hydrogen (see, e.g., T. Udem et al. In xe2x80x9cPhysical Review Letters,xe2x80x9d Vol. 79, 1997, pp. 2646 et seq.). These frequency standards possess;, however, optical frequencies above 80 THz, which can no longer be counted out directly electronically. For an optical clock, one needs therefore an apparatus for frequency transformation from the high frequency of the frequency standard to a low frequency that can be evaluated through electronic means. Such an apparatus possesses the function of a xe2x80x9cclockworkxe2x80x9d for an xe2x80x9coptical clock.xe2x80x9d
Harmonic frequency chains are used for the bridging of the large frequency separation between optical frequencies and. (electronically countable) radio frequencies (see H. Schnatz et al. In xe2x80x9cPhysical Review Lettersxe2x80x9d, Vol. 76, 1996, P. 18 et seq.). With a harmonic frequency chain, a reference frequency is multiplied with even number factors at a number of multiplication stages, until the desired frequency is attained. This requires, however, a separate transfer oscillator with a phase coupling to the preceding harmonic signal for each multiplication level. The availability of a numerous oscillators at various frequencies makes the setup voluminous, complicated, and expensive.
FIG. 13 illustrates a further principle of a well-known scaling stage for optical frequencies (see T. W. Hxc3xa4nsch in xe2x80x9cThe Hydrogen Atomxe2x80x9d, ed. G. F. Bassani et al., Springer Publishing, Berlin 1989, p. 93 et seq.; H. R. Telle et al. In xe2x80x9cOptics Letterxe2x80x9d, Vol. 15, 1990, p. 532 et seq.; and T. W. Hxc3xa4nsch in xe2x80x9cPhysikalische Blxc3xa4tter,xe2x80x9d Vol. 54, 1998. P. 1007 et seq.). If one overlaps two laser beams that differ only slightly in frequency onto a photo detector, one observes a modulation of light intensity at the differential frequency (beat signal). This beat signal may be used to adjust the frequency of one partial beam onto the frequency of the other partial beam. According to the scheme of FIG. 13, two laser frequencies f1 and f2 are compared to a third laser frequency f3 near the middle frequency (f1+f2/2). The summation frequency f1+f2 is produced with a nonlinear crystal (+), and the upper harmonic vibration 2 f3 is produced with a further nonlinear crystal (xc3x972). The lower frequency beat signal at the photo detector is used in the digital control loop "PHgr" to control the frequency and phase of the third laser to oscillate precisely on the middle frequency, i.e. f3=(f1+f2)/2. Thus a frequency interval xcex94f is reducible by a factor xc2xdn with a chain of n scaling stages according to FIG. 13. If one begins such a chain of scaling stages with a laser frequency f and its second upper harmonic 2f, that is, with xcex94f=f, then one obtains a differential frequency of f/2n after n scaling stages. The problem with the described scaling chains is that at least twelve scaling stages are needed to bridge the frequency gap between optical frequencies (fopt greater than 300 THz) and radio frequencies (fradio less than GHz) This presents an apparatus requirement that is unacceptably high for routine applications.
Using an optical frequency comb generator (OFC) has been suggested to reduce the number of required scaling stages required (see K. Imai et al. in xe2x80x9cIEEE Journal of Quantum Electronics,xe2x80x9d Vol. 34, 1998, p. 54 et seq). In an OFC, side bands on an optical carrier frequency are produced with a resonator adapted for optical and microwave frequencies, which are separated by a distance determined by a given microwave frequency. For the given frequency gap approx. five or six scaling stages are still necessary even with the use of an OFC. Details of an OFC are also explained in the named publication of T. W. Hxc3xa4nsch in xe2x80x9cPhysikalische Blxc3xa4tter,xe2x80x9d 1998.
With a view to the creation of a xe2x80x9cclock workxe2x80x9d for optical frequency standards, there is an interest in an optical frequency generator in which particularly large frequency differences are bridged with a relative accuracy that is significantly better than 10xe2x88x9214 and in particular makes a decrease in the number of scaling stages possible.
The second application of optical frequency generators in spectroscopy concerns the highly accurate frequency measurement of the light of a spectroscopy laser. The absolute frequency of, for example, the D1 resonance line of cesium could until now only measured with a relative accuracy of approx. 10xe2x88x927 (See K. H. Weber et al. in xe2x80x9cPhysical Review Axe2x80x9d, Vol. 35, 1987, p. 4650). There is an interest in the increasing of accuracy of frequency measurement of electronic states.
It is the object of the invention to provide a new process for the operation of a pulse laser with defined pulse parameters, in particular with defined mode positions, which makes possible a significant increase in accuracy in the production or measurement of optical frequencies and/or optical differential frequencies. In particular, the possibility of stabilizing the operation of a pulse laser is to be provided with this process, which can be implemented as a simple, reliable, quickly functioning and accurate means of regulation. It is also an object of the invention to provide a method for the generation or measurement of optical frequencies and/or optical differential frequencies. The object of the invention also consists of providing a device for the implementation of the process. Further, new applications of optical frequency generators are to be given with the invention.
These objects are solved with the processes and devices having the characteristics as set forth in the appended claims. Advantages embodiments and applications of the invention are defined in the dependent claims.
In accordance with a first aspect of the invention, a method for the operation of a pulse laser is given, in which light pulses circulating in a resonator configuration are produced, whereby a predetermined setting of various phase shifts for various spectral components of the light pulses is provided. Through the introduction of a linear dispersion (first order dispersion), at least one mode obtains a certain frequency and/or the mode distance between the modes is set to a certain value. It can be provided in particular that each mode is subject to a spectrally specific frequency change. The setting of spectrally different effective resonator lengths can follow through various measures for the introduction of a linear dispersion into the resonator. Among these is a geometric, spectrally specific resonator length setting in a resonator part, in which the pulses circulate spatially separated, the introduction of material with a linear dispersion into the resonator configuration and/or measures for setting of the pumping power at the laser medium. The portion of the resonator in which the pulses circulate in a spatially separated manner is arranged on the side of an apparatus for the compensation of the dispersion of group velocity (pulse compressor) which is turned away from the laser medium of the pulse laser. The introduction of a linear dispersion is to be differentiated from the compensation or setting of a group velocity dispersion (second order dispersion), which is necessary for pulse production.
Pursuant to a particular embodiment, the method of the invention is carried out as a control method for the stabilization of the operation of a pulse laser, in which the phase circulation time of the light pulse is varied dependent upon a frequency deviation of at least a first reference mode from a reference frequency. The pulse laser is stabilized with a mode control loop in relation to the reference frequency and with a repetition frequency control loop in relation to a frequency normal. The control loops work together for the adjustment of the pulse repetition frequency fr and the mode positions.
The pulse laser possesses a dispersion setting device for the adjustment of the linear dispersion in the resonator and a resonator length setting device for the adjustment of the resonator length. On the basis of the working together of both control loops described in detail below, it is provided that either the dispersion setting device in the mode control loop and the resonator length setting device in the repetition frequency control loop or, in reverse, the resonator length setting device in the mode control loop and the dispersion setting device in the repetition frequency control loop, be regulated.
According to a further embodiment of the invention, these control loops can work together with a reference laser control loop, with which the reference frequency is stabilized in relation to a second reference mode of the frequency comb.
According to a further embodiment, the inventive method is carried out as a control method for the operation of a reference laser with a stabilized reference frequency. The reference laser is regulated dependent upon a frequency deviation of the reference frequency (or its even number fractionals or multiples) of at least one reference mode of the light pulses circulating in the pulse laser, stabilized in relation to a frequency normal. The entire system with the reference laser makes up an inventive optical frequency synthesizer.
According to a second aspect of the invention, a laser device for the production of short light pulses is given, in which a resonator configuration with an active medium, a number of resonator mirrors and compensatory device for the compensation of the group velocity dispersion of the light pulses is provided, whereby the resonator configuration has at least one adjustable dispersion setting device for the adjustment of the phase circulation time of the light pulses according to the measures described above. The adjustment of the phase circulation time is provided preferably as a control simultaneous to the control of the repetition frequency with use of the mode and repetition frequency control loop.
The dispersion setting device is preferably provided in a branch of the resonator on the side of the compensation apparatus opposite relative to the active medium, for example, in the form of a tilting mechanism at a resonator end mirror. Alternatively, the dispersion setting device can be a transparent plate that can be angled or an insertable pair of prisms or also formed as an apparatus for the variation of the pumping power for the laser medium.
According to a further important aspect of the invention, a broad frequency comb is generated with a stabilized pulse laser device according to the invention, which frequency comb contains the first reference mode in a lower frequency range, whereby the second reference mode in a higher frequency range is used for adjusting the reference frequency of the mode control loop in the frame of a (third) reference laser control loop.
According to a further important aspect of the invention, a laser device is stabilized through the use of a frequency normal, preferably of a radio frequency source, so that the laser device and/or a reference laser coupled to it forms a generator of optical frequencies (optical frequency synthesizer) for precision applications in time or frequency measurement technology.